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Bulkhead Design for AMD Page 13 October 27-29, 1998 <br />i.e. opening of the joints and fractures and injection of acid mine <br />water into the rock mass around the plug and possibly to the ground <br />surface. <br />The hydraulic breakdown pressure (BP) available to hydrofrar_ <br />the rock immediately upstream from the plug and adjacent to the <br />tunnel is the maximum potential head. Therefore, the overburden <br />stress must be sufficient to prevent hydrofracing. The required <br />overburden stress (So„b) is: <br />Bp <br />S ovb - 2 <br />(23) <br />The overburden pressure is the product of the depth (H) and <br />density (y) of the overlying rock. Since the density can be <br />readily measured, the depth of the bulkhead must be selected to <br />limit the possibility of hydrofracing, as follows: <br />Sovb = 144 - 6P (24) <br />y = density (PCF) <br />H = depth (ft) <br />H = ~ 1BV (25) <br />Corrosion Resistant Design <br />The useful life of a concrete bulkhead is controlled by the <br />corrosive nature of the acid mine drainage being impounded, the <br />formulation of the concrete mix and on the corrosion resistance of <br />the piping penetrating through the bulkhead. The corrosion <br />characteristics of the impounded acid mine drainage can not be <br />controlled. It is likely that the quality of the drainage water <br />will change during the course of mine filling and after the maximum <br />head has been reached. Sampling of the water impounded immediately <br />behind the American Tunnel bulkhead has shown wide pH fluctuations <br />since the valve was closed. Initially the pH rose well above 8, <br />apparently as the result of the 20 tons of lime and 20 tons of <br />limestone placed upstream from the bulkhead. The pH has dropped to <br />2.8 in the last year possibly as the result of solutioning of <br />precipitates that have accumulated on the walls of underground <br />openings that have since been inundated. However, no iron has been <br />detected in the water samples taken at the American Tunnel <br />Bulkhead. Iron present as Fe" ions tend to surface coat limestone <br />limiting its further dissolution (USBM, 1994). <br />- 13 - <br />